Catalysts for up-grading steam-cracking products

ABSTRACT

There is provided with the present invention a new process for up-grading products resulting from the steam-cracking of hydrocarbons which comprises bringing the steam-cracking reaction products in contact with a catalyst comprising a mixture of from 2.5 to 7.5% wt of Cr 2  O 3 , 5 to 17.5% wt of Al 2  O 3  and 75 to 85% wt of a Zn-ZSM-5 zeolite or a Zn-ZSM-5 zeolite/asbestos and recovering the desired products.

BACKGROUND OF THE INVENTION

Steam-cracking is one of the most widely used basic petrochemicalprocesses. It is used by industries to produce light olefins such asethylene, propylene, butenes and butadiene and it is also relied uponfor the production of aromatics such as benzene, toluene and xylenes.

Basically, steam-cracking comprises a step in which the hydrocarbonmixture to be transformed is mixed with steam and submitted to elevatedtemperatures in a tubular reactor. The different resulting products,gaseous and liquid hydrocarbons are then collected and separated. Thus,product distribution depends on the nature of the initial hydrocarbonmixture as well as experimental conditions.

Among the products obtained, C₂ -C₄ light olefins, as well as benzene,toluene, ethylbenzene and xylenes have the highest commercial values andsince enormous quantities are processes throughout the world, even smallyield improvements lead to substantial profit increases.

In recent years, ZSM-5 zeolite catalysts have drawn considerableattention because of their ability to increase selectivity in a varietyof industrial processes such as xylene isomerization, toluenedisproportionation, aromatic alkylation and methanol conversion.

It has been shown that the zeolite's selectivity properties are theresult of its tridimensional channel network and of the different poresizes of its structure.

One of the most interesting areas where ZSM-5 zeolites have shownsubstantial catalytic activity is in the process in which methanol isconverted into hydrocarbons. Thus, by using appropriate reactionconditions, very high yields in C₅ -C₁₁ gasoline hydrocarbons can beobtained. However, this reaction presents the drawback of producingsmall quantities of durene, an undesirable reaction product.

Furthermore, modifications of the catalyst can also lead to highlyefficient production of light olefins resulting from methanolconversion.

Thus, it can be seen that modified zeolite catalysts have thepossibilities to present very interesting properties for enhancingyields in petrochemical reactions.

Therefore, since steam-cracking is one of the most widespreadpetrochemical processes, it would be highly desirable to provide meansfor increasing production of the most valuable reaction products.

SUMMARY OF THE INVENTION

The present invention relates to a process for up-grading productsresulting from the steam-cracking of hydrocarbons which comprisesbringing the steam-cracking reaction products in contact with amultifunctional Zn-ZSM-5 zeolite/Cr₂ O₃ /Al₂ O₃ catalyst comprising of amixture of from 2.5 to 7.5% wt of Cr₂ O₃, 5 to 17.5% wt of Al₂ O₃ and 75to 85% wt of a Zn-ZSM-5 zeolite or a Zn-ZSM-5 zeolite/asbestos. Such aprocess allows for significant yield increases in C₂ -C₄ olefins.Furthermore, the commonly obtained pyrolysis oil is up-graded to a highgrade gasoline, rich in mono-aromatics and free from undesirable durenesand long aliphatic chains.

DETAILED DESCRIPTION OF THE INVENTION

The main feature of the present invention resides in the presence of acatalytic reactor at the outlet of the steam-cracking reactor. Thiscatalytic reactor contains a multifunctional catalyst which comprises azeolite of the ZSM-5 type coupled with metallic oxides.

These oxides can either be coupled to the zeolite by being directlydeposited on the zeolite or mechanically mixed with the zeolite.

The metallic oxides can be selected from oxides such as Cr₂ O₃, Al₂ O₃,or from any metallic oxide having a hydrogenating/dehydrogenatingfunction.

In the case of the Cr₂ O₃, Al₂ O₃ proportions of Cr₂ O₃ ranging between2.5 and 7.5% wt, proportions of Al₂ O₃ ranging between 5 and 17.5% wtand proportions of the zeolite catalyst ranging between 75 and 85% wtcan be used.

Although the catalytic reactor used in the present invention was afixed-bed reactor, it will be understood that any suitable designcommonly used for catalytic reactions could have been chosen.

In the drawings:

FIG. 1 represents a schematic drawing of the bench scale setting for thecatalytic up-grading of products resulting from the steamcracking ofhydrocarbons.

FIG. 2 represents a comparison between the amounts of C₂ -C₄ olefinsobtained by steam-cracking alone and by steam-cracking along withvarious zeolite catalysts.

FIG. 3 represents a comparison between the amounts of ethylene obtainedby steam-cracking alone and by steam-cracking along with various zeolitecatalysts.

Referring now to FIG. 1, the starting hydrocarbon material 2 is firstmixed with a stripping gas 4. It is to be noted, however, that the useof a stripping gas is optional. In the context of the actualexperiments, a stripping gas was used only for convenience.

The resulting mixture is then forwarded to a vaporizer-mixer 6, in whichsteam is injected by means of an infusion pump 8. The gaseous mixturethus obtained enters a steam-cracking tubular reactor 10 heated at atemperature ranging between 760° and 860° C. In a further step, productscoming out of the steamcracking tubular reactor 10 are sent into acatalytic reactor 12 heated at a temperature ranging between 450° and550° C. The resulting products are then cooled by a series of condensers14 (water-cooling condensers and ice bath). Immediately following thecooling step, the liquid and gaseous phases are separated. The liquidsare first collected in a liquid-collector cylinder 16 while the gasesflow through the liquid-collector cylinder to be collected for on lineanalysis in a dynamic sampler cylinder 18 located at a higher positionthan the liquid collector cylinder.

The present invention will be more readily understood by referring tothe following examples which are given to illustrate rather than limitthe scope of the invention.

EXAMPLE 1

Propane is the starting hydrocarbon material on which the steam-crackingprocess was performed. It was introduced into the system at a flow rateof 45 ml/min. or 4.95 g/hour. It was first mixed with helium acting as astripping gas. After having been flown through the vaporizer-mixer, inwhich steam was injected at a rate of 1.7 g/hour, the gaseous mixturewas then sent into the steam-cracking reactor whose internal temperaturehad been set to 780° C. at atmospheric pressure. The residence time ofthe starting material in the steam-cracking reactor was approximately 1second.

The resulting product was then separated into its liquid and the gaseousphases. The liquid fraction was analyzed by GC using a capillary column(length: 50 m, PONA® type, fused silica coated with a crosslinkedpolymer). The gases were analyzed on line by gas chromatography. Acolumn having a length of 3.5 m packed with Chromosorb® P coated with20% by weight of Squalane® was used for the analysis. The GC used was adual FID Hewlett-Packard Model 5790 equipped with a 3392A Modelintegrator. Results are shown in Table 1.

EXAMPLE 2

The same procedure as in Example 1 was repeated the only modificationbeing the internal temperature of the steam-cracking reactor which wasset at 800° C. Results are shown in Table 1.

EXAMPLE 3

The same procedure as in Example 1 was repeated the only modificationbeing the internal temperature of the steam-cracking reactor which wasset at 835° C. Results are shown in Table 5.

EXAMPLE 5

As in Example 1 propane was chosen as the starting hydrocarbon material.It was mixed with helium and flown through the vaporizer-mixer. Thegaseous mixture was then forwarded through the steam-cracking reactorwhose internal temperature had been set to 780° C. The resultingproducts were then sent to the catalytic reactor which had beenpreviously embedded with 4 g of a Zn-Mn-ZSM-5 zeolite which was preparedaccording to the procedure described in Can. patent application Ser. No.471,463. The temperature of the catalytic reactor had been previouslyset at 500° C., with a pressure of 1 atmosphere and a W.H.S.V. (weighthourly space velocity) of 1 h⁻¹. The final products were analyzed usingthe techniques discussed in Example 1. Results are shown in Table 2.

EXAMPLE 5

The same procedure as in Example 4 was repeated, the only modificationbeing the internal temperature of the steam-cracking reactor which wasset at 800° C. Results are shown in Table 2.

EXAMPLE 6

The same procedure as in Example 4 was repeated, except for thefollowing modifications: the catalytic reactor was embedded with 4 g ofa Zn-Mn-ZSM-5 zeolite/asbestos catalyst prepared according to theprocedure described in Can. patent application Ser. No. 471,463. Resultsare shown in Table 3.

EXAMPLE 7

The same procedure as in Example 6 was repeated, the only modificationbeing the internal temperature of the steam-cracking reactor which wasset at 800° C. Results are shown in Table 3.

EXAMPLE 8

The same procedure as in Example 4 was repeated, except for thefollowing modification: the catalytic reactor was embedded with aZn-ZSM-5 zeolite/asbestos/Cr₂ O₃ /Al₂ O₃ catalyst. The Zn-ZSM-5zeolite/asbestos catalyst was prepared according to the method describedin Can. patent application Ser. No. 471,463 or related Le Van Mao U.S.Pat. No. 4,615,995. Then, 4.5 g of the Zn-ZSM-5 zeolite/asbestoscatalyst obtained were wet with a solution prepared from 0.3 g of Cr₂ O₃and 0.4 g of sodium aluminate dissolved in 5 ml of distilled water. Theresulting multifunctional catalyst was dried at 120° C. for 12 hours andactuated at 500° C. for another 12 hour period. Finally, the catalystwas reduced in hydrogen at 350° C. for at least 1 hour. Results areshown in Table 4.

EXAMPLE 9

The same procedure as in Example 8 was repeated, the only modificationbeing the internal temperature of the steam-cracking reactor which wasset at 800° C. Results were shown in Table 4.

When studying the results obtained from the various examples, it is tobe noted that in the steamcracking process alone (Table 1) significantincreases in highly valuable compounds such as ethylene, benzene andtoluene are observed when the internal temperature of the reactor isincreased from 780° to 800° C. The amount of less valuable products suchas methane is higher at 800° C. but this increase is compensated by adecrease in C₂ -C₄ paraffins.

As for the aromatic content, there is a dramatic decrease in lessvaluable C₅ -C₁₁ aliphatics, resulting in the obtention of moreinteresting products such as benzene, xylenes and toluene. In examples 4to 7 Zn-Mn-ZSM-5 zeolite and Zn-Mn-ZSM-5 zeolite/asbestos, two knowncatalysts were used to form the catalytic bed. As it can be seen inTables 2 and 3, and in FIGS. 2 and 3, inferior results were obtainedwhen compared to steam-cracking alone as far as the olefin content isconcerned, regardless of the temperature at which the reactions wereperformed.

As for the aromatic content, better results were obtained, but theseresults are at the best sufficient and no more, to compensate thequality loss on the side of the olefin production, especially, as far asethylene is concerned, since ethylene is the most valuablesteam-cracking product.

Thus, in the light of these results, one could tend to be led away fromusing zeolite catalysts as means to improve steam-cracking processes.

In Examples 8 and 9, the results obtained by using a multifunctionalcatalyst point out better results in both olefin and aromaticproductions. Thus, it has been discovered as it can be seen in FIGS. 2and 3, that the use of metal oxides co-catalyst coupled with a zeolitetype catalyst unexpectedly increases the amounts of valuablesteam-cracking products. In fact, the total amount of C₂ -C₄ olefins andespecially ethylene obtained by using the multifunctional catalyst aftera steam-cracking reaction of 780° (55.8% wt) is even superior to theamount obtained when performing the steam-cracking reaction alone at800° (47.1 wt).

Moreover as described in Example 3, a run without catalyst was performedat 835° C. This temperature was fairly close to temperatures used inindustrial steam-cracking facilities using propane as a startinghydrocarbon material. When the product distribution of such a run iscompared to the run performed in presence of the Zn-ZSM-5zeolite/asbestos/Cr₂ O₃ /Al₂ O₃ catalyst and with the steam-crackingreactor temperature set at 800° C., as described in Example 9, it can beseen, as it is shown in Table 5, that in the presence of themultifunctional catalyst and with a much lower steam-crackingtemperature, higher yields in ethylene and propylene were obtained. Thepropylene yield was nearly doubled (due mainly to a lower steam-crackingtemperature) and the ethylene yield was increased by 5 wt percentagepoints while methane formation was significantly lower.

Furthermore, the liquid yield was much lower for the run performed at alower steam-cracking temperature in the presence of the multifunctionalcatalyst. However, the BTX aromatics (benzene, toluene, ethylbenzene andxylenes) content in the liquid hydrocarbon products was much higher andthere was no formation of undesirable hydrocarbons.

Thus, by performing the steam-cracking of propane at a lower temperatureand using a multifunctional catalyst, the total "ethylene+propylene"yield can be increased by 10 wt percentage points and theethylene/propylene wt ratio can be decreased to a very large extent (seeTable 5).

From an industrial viewpoint, this would represent a real advantagesince the present market trends are for a lower demand in ethylene andan increasing demand in propylene.

It will be appreciated that even though yields increase in valuableproducts ranging from 5 to 10% wt do not seem to be of significantimportance, because of the enormous amounts of hydrocarbon materialrefined every day throughout the world, even a 0.5% wt yield increaserepresents millions of dollars of profits for petrochemical industries.Therefore, it is submitted that every invention increasing productionyields in the petrochemical conversion processes has tremendouscommercial values for these industries.

It will also be understood that although the process of the presentinvention has been developed for up-grading steam products, it can alsobe applied to every situation where starting materials such as pyrolysisoil, pyrolysis gasoline, mixtures of light olefins, light paraffins ormixture thereof are flown directly into the catalytic reactor withoutrequiring any passage through a steam-cracking reactor.

In such cases, the multifunctional properties of the catalyst areexpressed through several actions such as acid-catalyzed reactions(cracking, oligomerization, isomerization, transmutation) and redoxreactions on intermediates leading to the final products or on theproducts themselves.

Therefore, in the framework of the present invention, the reactorcontaining the multifunctional catalyst can be located either after thesteam-cracking reactor or after the liquid/gases separation operation(thus intercepting the liquid or gaseous products) and still obtainsimilar end results.

                                      TABLE 1                                     __________________________________________________________________________    DISTRIBUTION OF STEAM-CRACKING PRODUCTS OBTAINED IN THE ABSENCE OF            CATALYST                                                                                          Reaction Temperature           Reaction Temperature                           °C.                     °C.                                     780%  800%                     780%  800%                 Gaseous      Commercial                                                                           wt    wt    Liquid    Commercial                                                                             wt    wt                   Products     Value  Obtained                                                                            Obtained                                                                            Products  Value    Obtained                                                                            Obtained             __________________________________________________________________________                                    LIQUID YEILD (wt %)                                                                              .sup. .sup.  2.9.sup.(1                                                             )                    ETHYLENE     Highest                                                                              31.9  37.5  BENZENE   Highest  0.0   0.4                  PROPYLENE    ↓                                                                             11.8  10.9  XYLENES   ↓ 0.0   0.1                  (BUTENES +   ↓                                                                              3.4   3.5  ETHYL-    ↓ 0.0   0.0                  BUTADIENE)   ↓           BENZENE   ↓                            (C.sub.2 + C.sub.4 PARAFFINS                                                               ↓                                                                             16.3  11.6  TOLUENE   ↓ 0.0   0.2                  PROPANE      ↓                                                                             16.7  10.7  C.sub.9 -C.sub.10                                                                       ↓ 0.0   0.5                  METHANE      Lowest 17.7  22.9  AROMATICS ↓                            TOTAL OLEFIN YEILD (wt %)                                                                         47.1  51.9  C.sub.5 -C.sub.11                                                                       Lowest   2.2   1.7                  PROPANE CONVERSION (wt %)                                                                         83.3  89.3  ALIPHATICS                                                                     TOTAL AROMATIC CONTENT                                                                          0.0   42                   __________________________________________________________________________     .sup.(1) Presence of hydrocarbons heavier than C.sub.11                  

                                      TABLE 2                                     __________________________________________________________________________    COMPARISON OF DISTRIBUTION OF STEAM-CRACKING PRODUCTS                         OBTAINED IN THE ABSENCE OF CATALYST                                           AND IN THE PRESENCE OF Zn--Mn--ZSM-5 ZEOLITE CATALYST                         __________________________________________________________________________                   Steam-Cracking Temperature (°C.)                                       780% wt                                                                            780% wt  800% wt                                                                            800% wt                                                    obtained                                                                           obtained obtained                                                                           obtained                                    Gaseous Commercial                                                                           no   Zn--Mn--ZSM-5                                                                          no   Zn--Mn--ZSM-5                               Products                                                                              Value  catalyst                                                                           catalyst catalyst                                                                           catalyst                                    __________________________________________________________________________    ETHYLENE                                                                              Highest                                                                              31.9 22.9     37.5 32.8                                        PROPYLENE                                                                             ↓                                                                             11.8 12.1     10.9 10.7                                        (BUTENES +                                                                            ↓                                                                              3.4  8.7      3.5  6.3                                        BUTADIENE)                                                                    (C.sub.2 +C.sub.4                                                                     ↓                                                                             16.3 15.2     11.6 10.4                                        PARAFFINS)                                                                    PROPANE ↓                                                                             16.7 14.4     10.7  8.6                                        METHANE Lowest 17.7 19.7     22.9 24.4                                        PROPANE CONVERSION                                                                           83.3 85.6     89.3 91.4                                        (wt %)                                                                        TOTAL OLEFIN   47.1 43.7     51.9 49.8                                        YIELD (wt %)                                                                  __________________________________________________________________________                   Stream-Cracking Temperature (°C.)                                      780% wt                                                                            780% wt  800% wt                                                                            800% wt                                                    obtained                                                                           obtained obtained                                                                           obtained                                    Liquid  Commercial                                                                           no   Zn--Mn--ZSM-5                                                                          no   Zn--Mn--ZSM-5                               Products                                                                              Value  catalyst                                                                           catalyst catalyst                                                                           catalyst                                    __________________________________________________________________________    LIQUID YIELD (wt %)                                                                           .sup.  2.2.sup.(1)                                                                .sup.  7.1.sup.(2)                                                                     .sup.  2.9.sup.(1)                                                                 .sup.  6.8.sup.(2)                          BENZENE Highest                                                                              0.0  0.7      0.4  0.7                                         XYLENES ↓                                                                             0.0  0.9      0.1  0.8                                         ETHYL-  ↓                                                                             0.0  0.4      0.0  0.5                                         BENZENE                                                                       TOLUENE ↓                                                                             0.0  1.0      0.2  1.2                                         C.sub.9 -C.sub.10                                                                     ↓                                                                             0.0  1.1      0.5  0.9                                         AROMATICS                                                                     C.sub.5 -C.sub.11                                                                     Lowest 2.2  3.0      1.7  2.7                                         ALIPHATICS                                                                    TOTAL AROMATIC 0.0  58       42   61                                          Content (%)                                                                   __________________________________________________________________________     .sup.(1) Presence of hydrocarbons heavier than C.sub.11                       .sup.(1) Absence of hydrocarbons heavier than C.sub.11                   

                                      TABLE 3                                     __________________________________________________________________________    COMPARISON OF DISTRIBUTION OF STEAM-CRACKING PRODUCTS                         OBTAINED IN THE ABSENCE OF CATALYST AND                                       IN THE PRESENCE OF Zn--Mn--ZSM-5 ZEOLITE/ASBESTOS CATALYST                    __________________________________________________________________________                   Steam-Cracking Temperature (°C.)                                            780% wt       800% wt                                                         obtained      obtained                                                   780% wt                                                                            Zn--Mn--ZSM-5                                                                          800% wt                                                                            Zn--Mn--ZSM-5                                              obtained                                                                           zeolite/ obtained                                                                           zeolite/                                    Gaseous Commercial                                                                           no   asbestos no   asbestos                                    Products                                                                              Value  catalyst                                                                           catalyst catalyst                                                                           catalyst                                    __________________________________________________________________________    ETHYLENE                                                                              Highest                                                                              31.9 22.7     37.5 31.5                                        PROPYLENE                                                                             ↓                                                                             11.8 11.6     10.9 11.3                                        (BUTENES +                                                                            ↓                                                                              3.4  8.4      3.5  8.2                                        BUTADIENE)                                                                    (C.sub.2 +C.sub.4                                                                     ↓                                                                             16.3 16.5     11.6 10.3                                        PARAFFINS)                                                                    PROPANE ↓                                                                             16.7 16.8     10.7  8.3                                        METHANE Lowest 17.7 18.4     22.9 23.8                                        PROPANE CONVERSION                                                                           83.3 83.2     89.3 91.7                                        (wt %)                                                                        TOTAL OLEFIN   47.1 42.7     51.9 51.0                                        YIELD (wt %)                                                                  __________________________________________________________________________                   Steam-Cracking Temperature (°C.)                                                          800% wt                                                         780% wt       obtained                                                   780% wt                                                                            obtained 800% wt                                                                            Zn--Mn--ZSM-5                                              obtained                                                                           Zn-- Mn--ZSM-5                                                                         obtained                                                                           zeolite/                                    Liquid  Commercial                                                                           no   zeolite/ no   asbestos                                    Products                                                                              Value  catalyst                                                                           asbestos catalyst                                                                           catalyst                                    __________________________________________________________________________    LIQUID YIELD (wt %)                                                                          .sup.  2.2.sup.(1)                                                                 .sup.  5.7.sup.(2)                                                                     .sup.  2.9.sup.(1)                                                                 .sup.  6.7.sup.(2)                          BENZENE Highest                                                                              0.0  0.4      0.4  0.4                                         XYLENES ↓                                                                             0.0  0.5      0.1  0.7                                         ETHYL-  ↓                                                                             0.0  0.4      0.0  0.4                                         BENZENE                                                                       TOLUENE ↓                                                                             0.0  0.9      0.2  1.2                                         C.sub.9 -C.sub.10                                                                     ↓                                                                             0.0  1.0      0.5  1.2                                         AROMATICS                                                                     C.sub.5 -C.sub.11                                                                     Lowest 2.2  2.5      1.7  2.8                                         ALIPH.                                                                        TOTAL AROMATIC 0.0  56       42   58                                          CONTENT (%)                                                                   __________________________________________________________________________     .sup.(1) Presence of hydrocarbons heavier than C.sub.11                       .sup.(2) Absence of hydrocarbons heavier than C.sub.11                   

                                      TABLE 4                                     __________________________________________________________________________    COMPARISON OF DISTRIBUTION OF STEAM-CRACKING PRODUCTS                         OBTAINED IN THE ABSENCE OF CATALYST AND IN THE PRESENCE                       OF Zn--ZSM-5 ZEOLITE/ASBESTOS/Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3              __________________________________________________________________________    CATALYST                                                                                     Steam-Cracking Temperature (°C.)                                            780% wt     800% wt                                                           obtained    obtained                                                          Zn--ZSM-5   Zn--ZSM-5                                                    780% wt                                                                            zeolite/                                                                             800% wt                                                                            zeolite/                                                     obtained                                                                           asbestos/                                                                            obtained                                                                           asbestos/                                     Gaseous Commercial                                                                           no   Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                   no   Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3            Products                                                                              Value  catalyst                                                                           Catalyst                                                                             catalyst                                                                           Catalyst                                      __________________________________________________________________________    ETHYLENE                                                                              Highest                                                                              31.9 41.5   37.5 46.2                                          PROPYLENE                                                                             ↓                                                                             11.8 11.4   10.9  8.7                                          (BUTENES +                                                                            ↓                                                                              3.4  2.9    3.5  3.2                                          BUTADIENE)                                                                    (C.sub.2 + C.sub.4                                                                    ↓                                                                             16.3 10.5   11.6  6.9                                          PARRAFINS)                                                                    PROPANE ↓                                                                             16.7  8.2   10.7  4.0                                          METHANE Lowest 17.7 23.4   22.9 27.9                                          PROPANE CONVERSION                                                                           83.3 91.8   89.3 96.0                                          (wt %)                                                                        TOTAL OLEFIN   47.1 55.8   51.9 58.1                                          YIELD (wt %)                                                                  __________________________________________________________________________                   Steam-Cracking Temperature (°C.)                                            780% wt     800% wt                                                           obtained    obtained                                                          Zn--ZSM-5   Zn--ZSM-5                                                    780% wt                                                                            zeolite/                                                                             800% wt                                                                            zeolite/                                                     obtained                                                                           asbestos/                                                                            obtained                                                                           asbestos/                                     Liquid  Commercial                                                                           no   Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                   no   Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3            Products                                                                              Value  catalyst                                                                           Catalyst                                                                             catalyst                                                                           Catalyst                                      __________________________________________________________________________    LIQUID YIELD (wt %)                                                                          .sup.  2.2.sup.(1)                                                                 .sup.  2.1.sup.(2)                                                                   .sup.  2.9.sup. (1)                                                                .sup.  3.1.sup.(2)                            BENZENE Highest                                                                              0.0  0.2    0.4  0.4                                           XYLENES ↓                                                                             0.0  0.2    0.1  0.4                                           ETHYL-  ↓                                                                             0.0  0.1    0.0  0.3                                           BENZENE                                                                       TOLUENE ↓                                                                             0.0  0.2    0.2  0.7                                           C.sub.9 -C.sub.10                                                                     ↓                                                                             0.0  0.6    0.5  0.5                                           AROMATICS                                                                     C.sub.5 -C.sub.11                                                                     Lowest 2.2  0.8    1.7  0.8                                           ALIPH.                                                                        TOTAL AROMATIC 0.0  57     42   73                                            CONTENT (%)                                                                   __________________________________________________________________________     .sup.(1) Presence of hydrocarbons heavier than                                .sup.(2) Absence of hydrocarbons heavier than C.sub.11                   

                                      TABLE 5                                     __________________________________________________________________________    COMPARISON OF DISTRIBUTION OF STEAM-CRACKING PRODUCTS OBTAINED IN THE         ABSENCE OF CATALYST                                                           AT 835° C. AND IN THE PRESENCE OF Zn--ZSM-5 ZEOLITE/ASBESTOS/Cr.sub    .2 O.sub.3 /Al.sub.2 O.sub.3 CATALYST AT 800° C.                                        Steam-Cracking Temperature    Steam-Cracking                                                                Temperature                                     (°C.)                  (°C.)                                          800% wt                       800% wt                                         obtained                      obtained                                  835% wt                                                                             Zn--ZSM-5               835% wt                                                                             Zn--ZSM-5                                 obtained                                                                            zeolite/asbestos/       obtained                                                                            zeolite/asbestos/        Gaseous  Commercial                                                                            no    Cr.sub.2 O.sub.3 /Al.sub.2 O.sub.3                                                     Liquid         no    Cr.sub.2 O.sub.3                                                              /Al.sub.2 O.sub.3        Products Value   catalyst                                                                            catalyst Products       catalyst                                                                            catalyst                 __________________________________________________________________________    ETHYLENE Highest 41.2  46.2     LIQUID HYDROCARBON                                                                           7.8   3.1                      PROPYLENE                                                                              ↓                                                                              4.5   8.7      YIELD (wt %)                                  (BUTENES +                                                                             ↓                                                                              3.2   3.2      Total aromatic 78.4  73.1                     BUTADIENE)                      content (%)                                   (C.sub.2 + C.sub.4                                                                     ↓                                                                              7.7   6.9      BTX aromatic   48.3  58.2                     PARAFFINS)                      content (%)                                   PROPANE  ↓                                                                              6.1   4.0      Liquid hydrocarbons                                                                          4.0   0.0                      METHANE  Lowest  29.5  27.9     heavier than C.sub.11 (%)                     PROPANE CONVERSION                                                                             93.9  96.0                                                   (wt %)                                                                        (ETHYLENE + PROPYLENE)                                                                         45.7  54.9                                                   YIELD (wt %)                                                                  ETHYLENE/PROPYLENE                                                                             9.2   5.3                                                    wt ratio                                                                      __________________________________________________________________________

What is claimed is:
 1. A catalyst suitable for up-grading productsresulting from the steam-cracking of hydrocarbons, said catalystcomprising 2.5 to 7.5% wt of Cr₂ O₃, 5 to 17.5% wt of Al₂ O₃ and 75 to85% wt of a Zn-ZSM-5 zeolite or a Zn-ZSM-5 zeolite/asbestos.
 2. Thecatalyst of claim 1, wherein CR₂ O₃ and Al₂ O₃ are mechanically mixedwith the Zn-ZSM-5 zeolite or the Zn-ZSM-5 zeolite/asbestos.
 3. Thecatalyst of claim 1, wherein CR₂ O₃ and Al₂ O₃ are directly deposited onthe Zn-ZSM-5 zeolite or the Zn-ZSM-5 zeolite/asbestos.
 4. A catalystaccording to claim 1 consisting of 2.5 to 7.5% wt. of Cr₂ O₃, 5 to 17.5%st. of Al₂ O₃ and 75 to 85% wt. of an Zn-ZSM-5 zeolite or Zn-ZSM-5zeolite asbestos.
 5. The catalyst according to claim 4 wherein CR₂ O₃and Al₂ O₃ are mechanically mixed with the Zn-ZSM-5 zeolite or theZn-ZSM-5 zeolite/asbestos.
 6. The catalyst according to claim 4 whereinCR₂ O₃ and Al₂ O₃ are directly deposited on the Zn-ZSM-5 zeolite or theZn-ZSM-5 zeolite/asbestos.